Gene transfer of a therapeutic protein is a promising approach toward treatment of genetic disease. Adenoassociated viral (AAV) mediated in vivo transfer of a coagulation factor IX (F.IX) gene to muscle or liver of subjects with severe hemophilia B is currently being evaluated in clinical trials. A major concern of treatment is induction of CDS* cytotoxic T lymphocyte responses (CTL), which may cause elimination of transduced cells (muscle fibers or hepatocytes) expressing the transgene product. Interestingly, AAV gene transfer has been shown not to cause CTL responses to a number of transgene products, which may be due to immune tolerance or ignorance. We will determine requirements for tolerance induction to a human F.IX transgene product in mice, which will receive AAV-F.IX gene transfer followed by challenge with an adenoviral vector (a potent vector for induction of CTL) expressing the identical transgene. CTL responses will be measured by antigen-specific lysis of target cells and immunochemical analyses of transduced tissues. We will define requirements for tolerance induction (target organ, levels of transgene expression, systemic protein delivery). We hypothesize that AAV-mediated hepatic gene transfer resulting in systemic F.IX protein delivery can lead to immune tolerance thereby preventing CTL responses to F.IX. Furthermore, we hypothesize that endogenous expression of immunodominant F.IX T cell epitopes contributes to tolerance thereby reducing the risk of CTL responses. The effect of the underlying mutation in endogenously expressed F.IX on CTL responses will be tested in a series of transgenic mice expressing non-functional variants of human F.IX taken from the hemophilia B database. Finally, a series of experiments is;proposed to identify subsets of regulatory T cells that can suppress development of CTL responses to F.IX. These studies are based on the hypothesis that hepatic AAV-F.IX gene transfer induces regulatory CD4+ T cells capable of suppressing CTL to F.IX. A detailed characterization of these regulatory cells and the requirements for their induction will lead to improved gene transfer protocols and will be an experimental basis for approaches to augment regulatory T cell responses. Improved understanding of cellular mechanisms underlying immune responses or absence thereof should provide the basis for design of strategies that preclude destruction of cells receiving viral gene transfer.